1. Field of the Invention
The present invention relates generally to an automatic transmission for use in automotive vehicles and more specifically to a line pressure control arrangement for such a transmission which maintains an adequate amount of friction element engagement during modes of operation when a sudden reduction in engine load occurs.
2. Description of the Prior Art
An automatic transmission Service Manual (publication A261C07) issued in March 1987 by the Nissan Motor Company relating to the so called RE4RO1A type transmission, describes an arrangement which utilizes tabled data for controlling the line pressure in response to engine load during shifting and non-shifting modes of transmission operation.
In this arrangement in order to control the level of line pressure to one suited for the instant mode of operation and therefore obviate shift shock which tends to be induced by excessively high line pressures, the engine throttle valve opening degree is used to determine the duty cycle of a signal which is used to control a line pressure control solenoid valve.
However, with this arrangement, as the line pressure is controlled in direct response to the throttle valve position, a problem is encountered in that when a depressed accelerator pedal is suddenly released, the line pressure drops suddenly along with the amount of amount of torque which is transmitted through the transmission, to a level lower than actually desirable.
There are several notable examples wherein such a phenomenon induces undesirable transmission operational characteristics.
The first is such that under stall conditions, when the accelerator pedal is released, the intertia torque is larger than the torque actually being produced by the engine. Nevertheless, the level of line pressure is controlled in response to the throttle opening degree and is according suddenly reduced with the rapid closure of the throttle valve. The amount of engagement of the friction elements becomes lower than suited for handling the reverse flowing inertia torque and the jitter occurs.
The second is such that when, as shown in FIG. 6A, the throttle valve opening is moved from TH=0/8 such as indicated at time t1, to an opening of TH=4/8, and then subsequently closed again to TH=0/8, the engine torque rises and falls as indicated in FIG. 6B. However, the line pressure tends to closely parallel the throttle opening as indicated in the solid line trace in FIG. 6C. As a result, during the period To which is defined between t2 and t3, the torque smoothly decreases from a peak value while the line pressure drops to a level which is inadequate to handle the amount of torque being outpulled by the engine and as a result jitter is again encountered.
In a third instance (see FIGS. 9 and 11) when during stall conditions the accelerator pedal is released, the engine speed reduces as indicated in FIG. 9A, while the line pressure drops suddenly as indicated by the solid line trace in FIG. 9B. Accordingly, during the period the engine speed reduces relatively slowly, a relatively large amount of torque continues to be supplied from the torque converter to the transmission. Again under these conditions the amount of force with which the friction elements of the transmission are engage is reduced to a level which is inadequate for the instant set of operating conditions and jitter is produced.